The present invention relates to a contact detecting system in a machine tool for detecting the presence of a workpiece by contact herewith with a view to measuring of its position and size and furthermore centering the workpiece, used in combination with a contact detecting instrument, preferably of a touch sensor containing a switch responsive to the physical contact with the workpiece, secured in a spindle of the machine tool.
Known systems for detecting of an object such as a workpiece, which is applied to a machine tool such as a machining center or the like, ordinarily use a touch detecting instrument secured in the spindle to make contact with the object so as to complete or break an electrical circuit by the contact with the object. In a touch detecting instrument of such nature, there are generally known two types of instruments.
The first example is one referred to hereinafter as a touch probe assembly, or a tool having conductive quality such as a drill or a tap, which is made of a conducting material and functions as one part of a circuit itself to make a complete circuit through the contact with an object of conductor like metal material.
The second representative is the one in which a movable stylus is arranged so as to function as one component of an electrical switch, usually in an closed condition for completing an electrical circuit, but in an open condition for disconnecting the circuit when the stylus is brought into contact with an object or workpiece and hence moves itself. The second example is referred to as a switch-contained touch sensor hereinafter in the specification.
At first, the embodied system employing the above-mentioned touch probe assembly of the first example will be described in minute detail with reference to FIG. 3. The machine tool to which the system is applied comprises externally a spindle 2 and a body 4 of which a lower portion is formed as a table for a workpiece 3. The touch probe assembly 1 including a touch probe member like a protrudence which is made of a conductor is secured in the spindle 2, whereby the touch probe assembly 1 can be relatively moved toward and away from the workpiece 3 with a up-and-down motion of the spindle 2. The machine tool is provided furthermore with necessary components of the system. In particular, an annular electromagnetic sensor unit 6 for electromagnetically sensing an electrical circuit followed by an electric current is mounted on the periphery of the spindle 2, and a touch-signal issue circuitry 8 for feeding a touch-signal 7 indicative of the contact of the touch probe assembly 1 with the workpiece 3 is provided in the body of the machine tool. The system on the whole is so formed that the touch-signal 7 is fed by and from the touch-signal issue circuitry 8 to a NC (numerical control) apparatus 9 for controlling the movement of the spindle 2 when the electromagnetical sensor unit 6 electromagnetically senses a closed circuit 5 (passing the spindle 2, the touch probe assembly 1, the workpiece 3, the parts of the body 4 of the machine tool and the foregoing spindle 2) which is made depending on the contact of the touch probe with the workpiece 3 of conductor. The above-described system has, however, a disadvantage in that an object to be detected is limited only to workpieces of a conductor made of metal or the like by reason of utilizing the object as a part of the closed circuit 5.
Next, the other embodied system employing the above-mentioned switch contained touch sensor will be described in minute detail with reference to FIG. 4. The switch-contained touch sensor 12 secured in the spindle 2 of the machine tool contains the switch mechanism 11 insulated from an outside casing, wherein a housed inner-part of the movable stylus 10 functions as a constituent element of the switch mechanism so as to open the switch 11 which is closed usually for completing a circuit and then break the circuit when a bare outer part of the movable stylus is brought in contact with the workpiece 3 and hence moves itself. In order that a complete closed circuit 15, which passes the above-mentioned usually-closed-switch 11 and the body 4 of the machine tool, is formed when the touch sensor 12 is attached in the spindle 2, the casing of the touch sensor 12 is externally provided with a contact terminal 14, whereas the body 4 of the machine tool is provided with an associated terminal bar 13 of a conductor in opposition to the above-mentioned contact terminal 14 of the touch sensor 12. In addition, components in the circuit 15 are connected by wire 16, as needed so that the closed circuit 15 can be completed through one portion of the body 4 of the machine tool, the terminal bar 13, the contact terminal 14, the usually closed-switch 11 and the foregoing body 4.
The second system on the whole is so formed that the touch-signal 7 is fed by and from a touch-signal issue circuitry 17 when the electromagnetic sensor unit 6 surrounding the spindle 2 fails to electromagnetically sense the closed circuit 15 because the circuit 15 is broken by the movement of the movable stylus 10 depending upon the contact with the workpiece 3. The second system never utilizes the object of the workpiece as a part of the closed circuit 15, so that this system can be advantageously used with relation to every workpieces regardless of the qualities of materials including non-conductors, in contrast with the first system which can be used only with a workpiece made of a conductor.
The above-mentioned annular electromagnetic sensor unit 6 which is mounted on the outside of the spindle 2 and incorporated in both the above-mentioned systems comprises, as illustrated in FIG. 4, an exciting coil 18 wound around a ring-shaped core of a magnetizeable material like ferrite so as to cause an induced current to flow in the above-mentioned closed-circuit 5 or 15, a detecting coil 19 wound around the similar core so as to sense the closed circuit 5 or 15 through the generation of a subsequently induced current in oneself, and an annular shielding plate (not shown in the figure) positioned therebetween.
In the first system in which the touch probe assembly is incorporated, when the closed circuit 5 is made by the contact of the touch probe with the workpiece, the subsequently induced current arises in the above-mentioned detecting coil 19 of the sensor unit 6 and flows in the form of output to the touch-signal issue circuitry 8. Accordingly, it is possible to amplify the touch-signal 7 in the circuitry 8 by the utilization of the above-mentioned output.
On the other hand, in the second system in which the switch-contained touch sensor is incorporated, the circuit 15 followed by electric current is made usually, so that the above-mentioned detecting-coil 19 of the sensor unit 16 has a subsequently induced current generated therein. However, when the movable stylus 10 of the touch sensor 12 is brought into contact with the workpiece to open the contained switch 11, the circuit 15 is broken while the subsequent induced-current does not arise at all in the detecting coil 19. On such an occasion, it is necessary to feed the touch-signal 7 by and from the touch signal issue circuitry 17, but an output cannot be obtained from the detecting coil 19, with the result that it is impossible to utilize such an output as energy for amplifying the touch-signal 7 in the circuitry 17. Therefore, in the system using the switch-contained touch sensor, it is necessary to provide the touch-signal issue circuitry 17 internally with a device for amplifying the touch signal 7 so as to feed an appropriate magnitude of the touch-signal 7 from the circuitry 17 when the output from the detecting coil 19 disappears. Or, an operating method in the input side of the NC apparatus 9 arranged at the end is adapted so as to operate only by a low level of input signal.
In other words, as compared to the system employing the touch probe assembly 1, system employing the switch-contained touch sensor 12 has a disadvantage in requiring either exchange touch-signal issue circuitries 8, 17 or modification of an operating method in the input side of the NC apparatus, or else use together with a separate device for inversely changing a condition of the output from the detecting coil 19 of the sensor unit 16.